1. Field of the Invention
The present invention relates to a laser processing apparatus and a laser processing method. In the laser processing apparatus and the laser processing method, a substrate is allowed to move horizontally and is radiated with a laser beam in a state that a liquid is supplied onto a surface thereof, thus, the surface of the substrate is subjected to predetermined processing.
2. Description of the Background Art
Conventionally, there has been known the following process. That is, in a sequence of procedures for manufacturing a semiconductor device, a substrate such as a semiconductor wafer (hereinafter, simply referred to as “wafer”) or a glass substrate for a liquid crystal display is subjected to predetermined processing by means of a laser beam. For example, Japanese Patent Laying-Open No. 2002-224878 discloses a technique for scanning a surface of a substrate with a laser beam to thereby form a dicing line. Japanese Patent Laying-Open No. 2003-249427 discloses a technique for removing a resist film, that is previously formed on a wafer prior to implementation of exposure processing for the wafer and covers an alignment mark used as a reference for positioning of the wafer, from the wafer by radiation of a laser beam to thereby bare the alignment mark. Generally, a laser beam has large energy and can achieve highly accurate alignment. Therefore, the laser beam is suitably used in such processing.
When the resist film is removed from the surface of the substrate by radiation of the laser beam, substances to be processed, that is, cuttings of the film are attached to the surface of the substrate and the film around the processed portion is heated due to the laser beam, resulting in an influence on subsequent processing for the substrate. In order to suppress the influence due to attachment of the cuttings and heat to the film, a liquid such as pure water is supplied onto the surface of the substrate to form a liquid film and the surface of the substrate is radiated with a laser beam through the liquid film.
Japanese Patent Laying-Open No. 2003-249427 discloses a laser processing apparatus that radiates a laser beam to the surface of the wafer having the resist film formed thereon through a liquid film, removes the resist film from a target position (film removal position), and bares the alignment mark. Hereinafter, brief description will be given of a configuration of this laser processing apparatus with reference to FIGS. 18A and 18B. A chuck 11 for holding a wafer W is connected to a drive part (not illustrated) through a support part 12 and is movable horizontally. A main nozzle 13 ejects pure water, to which a pressure is applied by means of pressure applying gas such as N2 (nitrogen) gas, toward a target position 10 on wafer W. A guide member 14 disposed on target position 10 in laser processing plays a role of guiding a stream of a liquid supplied from main nozzle 13 and a stream of a liquid supplied from a sub nozzle (to be described later), and is connected to main nozzle 13 through a support member 14a. Herein, guide member 14 is made of, for example, quartz so as to allow a laser beam to transmit therethrough. A laser beam radiation part 16 radiates a laser beam to target position 10 as shown by a dot line in the figure.
Sub nozzles 17 and 18 are provided to guide member 14, respectively. Each of sub nozzles 17 and 18 is opened at a bottom face of guide member 14. As illustrated FIG. 18B, sub nozzles 17 and 18 are provided symmetrically with main nozzle 13 interposed therebetween when being seen from above. Each of sub nozzles 17 and 18 plays a role of ejecting pure water toward target position 10 at a flow rate slower than that of main nozzle 13 to thereby suppress extension of the stream of the liquid supplied from main nozzle 13. A liquid recovery part 19 sucks the pure water, that is supplied from each of nozzles 13, 17 and 18 and, then, passes below guide member 14, to remove the pure water from wafer W.
Wafer W is subjected to predetermined processing by the aforementioned laser processing apparatus in the following manner. First, guide member 14 is allowed to move to a position above target position 10. Then, pure water is ejected from each of nozzles 13, 17 and 18. On the other hand, liquid recovery part 19 sucks the pure water. A liquid stream 10a is formed between wafer W and guide member 14. Target position 10 is radiated with a laser beam by laser beam radiation part 16, so that a film on target position 10 is cut. Cuttings of the film are washed out by the pure water, and liquid recovery part 19 removes the cuttings and the pure water from wafer W.
A laser beam used herein is intermittently emitted in form of a pulse toward target position 10 for each 20 μsec, for example. This emission of the laser beam in form of a pulse causes the following disadvantage. That is, a component of the film at target position 10 is evaporated, and this evaporated component absorbs energy of the emitted laser beam in form of a pulse, so that plasma is generated at target position 10. This plasma abruptly compresses a surrounding liquid, so that air bubbles each having a diameter of about 1 mm are generated at target position 10. In a state that such air bubbles are generated at target position 10, a laser beam in form of a pulse is intermittently emitted toward target position 10 to thereby collide with the air bubble. As a result, the laser beam causes optical scattering, and is disadvantageously emitted to a portion other than target position 10. Consequently, the film on target position 10 is not processed satisfactorily, the film other than the film on the target position 10 is damaged, and delamination occurs at the damaged film upon reception of a pressure of liquid stream 10a. 
In order to prevent scattered reflection of the laser beam, a radiation position of wafer W of a laser beam and each nozzle for supplying the liquid are appropriately adjusted, so that air bubbles generated due to the plasma must be promptly removed from a target position by washout.
In order to sufficiently remove the cuttings of the film from wafer W by washout using a liquid stream in the laser processing apparatus, pure water to be ejected from main nozzle 13 must be applied with a pressure so as to secure a flow rate to a degree. If the flow rate of the pure water from main nozzle 13 is accelerated, the air bubbles generated due to the plasma can be promptly washed out from target position 10, so that optical scattering of the laser beam due to the air bubbles can be suppressed.
In a case that the flow rate is accelerated, however, a change in pressure to be applied to the pure water ejected from main nozzle 13 onto wafer W is considerable. As a result, so-called cavitation, in which gas dissolved in the pure water appear as micro bubbles in the liquid stream, readily occurs. In a case that a laser beam collides with the micro bubbles, optical scattering occurs as in the case that the laser beam collides with the air bubbles generated due to the plasma. As a result, target position 10 is not sufficiently radiated with the laser beam, and the portion other than target position 10 is disadvantageously radiated with the laser beam.
Neither Japanese Patent Laying-Open No. 2002-224878 nor Japanese Patent Laying-Open No. 2003-249427 discloses not only the aforementioned problems but also solution of the problems.